1. Field of the Invention
The present invention relates to a heterojunction transistor having a high mutual conductance and operable at a high speed.
2. Description of the Prior Art
As an active semiconductor device operable at a high speed, an FET (Field Effect Transistor) making use of two-dimensional electrons at a hetero-boundary surface between semiconductors, has been known (for instance, Japan Journal of Applied Physics 19 (1988) L255). This device is characterized in that, at a hetero-boundary between semiconductors having different electron affinities (for example, Al.sub.x G.sub.1-x As/GaAs), only the semiconductor having a smaller electron affinity is doped with donor impurities to generate two-dimensional electrons on the side of the semiconductor having a larger electron affinity, and in that a high mobility of these two-dimensional electrons is utilized. However, in view of an operation mechanism, this FET can be deemed as one kind of MISFET (Metal Insulator Semiconductor FET) in which a semiconductor having a wide energy gap is used in place of an insulator film, and hence it has similar advantages and shortcomings to a MOSFET (Metal-Oxide-Semiconductor FET). A MIS FET is easy to be integrated to a high degree of integration, because the process is short as compared to a bipolar transistor and a planar structure is easy to be manufactured. On the other hand, since a mutual conductance which represents a load driving capability of a device is lowered as the sizes of the elements are micro-fined, a proportion of delay caused by increase of a wiring capacitance associated with high integration and by driving of an external load, is increased. Accordingly, to enhance the speed of the overall system is not so easy as in the case of the bipolar transistors.
A bipolar transistor having a high load driving capability of the type that electrons are injected into two-dimensional electron gas, is disclosed in U.S. patent application Ser. No. 807,935 filed on Dec. 12, 1985 and assigned to the same assignee as this application. More particularly, on a semi-insulating substrate is formed a low impurity concentration GaAs layer, then an n-type Al.sub.x Ga.sub.1-x As layer in which x changes gradually from 0.3 to 0 is formed thereon, and on one part of the layer is formed a p-type GaAs layer. On this p-type GaAs layer is deposited a metallic gate electrode, and source and drain electrodes made of Au-Ge are deposited on the n-type GaAs layers on the opposite sides.
Two-dimensional electron gas is produced on the surface of the low impurity concentration GaAs layer and a transistor action is provided by modulating a source-drain conductivity relying upon this two-dimensional electron gas. Especially, the gate electrode is applied with such voltage that the junction between the p-type GaAs layer and the n-type GaAs layer is forward biased. Due to such gate bias, electrons are injected into the two-dimensional electron gas, and a source-drain conductivity is increased. Thereby, despite of a heterojunction transistor, a large load driving capability and a high speed operating characteristic similar to a bipolar transistor were obtained.
However, in such heterojunction field effect transistor in the prior art, while at the bottom of the conduction band in the energy band structure under he gate electrode, a recess accumulating electrons exists, a recess accumulating holes does not exist, at the top of valence band. Therefore, further improvement is desired in the load driving capability and the high speed operation characteristic.